Thanks to the new Cat in the Hat show on PBS, child 1 has shown a big interest in all things Dr. Seuss. We recently discovered Seussville.com where there are a number of games and activities to support such an interest. Favorites so far are the A-maze-ing Green Eggs and Ham game (which he is surprisingly really good at) and Horton hears a Tune (the Monkey See Monkey Do game). He also loves to create his own "who". There are also a number of printable coloring/activity pages that look fun (we've done a couple), and a few video clips.
This site seems like a good one for slightly older children too (like early elementary I'm guessing). Preschool would be on the low end of the ages that would be able to play the games and still find it fun.
Registration is not required to play anything, but if you do register (free) you can save your "who" and earn dooklas which you can use to get more who accessories. There may be other advantages to registration I am unaware of yet.